1. Field of the Invention
This invention relates to and has among its objects the provision of methods and apparatus for controlling the suspension of particles in a liquid medium. One aspect of the present invention is directed to a method of keeping cells in suspension in a liquid without the use of stirring or other high shear mixing or of the use of viscous or high specific gravity liquids. The method is particularly applicable to maintaining a suspension of erythrocytes intended for use in blood typing and grouping.
It is often desirable to keep particles suspended in a liquid in order to permit reproducible numbers of particles to be withdrawn, to transport the particles in a fluid stream, or to facilitate diffusion of reactants to the particles, such as, for example, diffusion of nutrients to cells. The present method provides for very efficient mixing of cellular suspensions and avoids the need for continuous agitation. Settling can be prevented by mechanical or magnetic stirring; bubbling a gas through the liquid; rocking, spinning or tumbling the container; pumping the liquid so as to cause a turbulent flow, etc. In general, these methods are problematic for long-term suspension of cells because they tend to cause gradual lysis.
Alternatively, cells can be suspended in liquids having high viscosity or a specific gravity similar to that of the cells. However, the use of these liquids can be undesirable because of adverse effects on cell stability and lifetime. Such liquids can also interfere with the intended purpose of maintaining the suspension, such as for use in an assay of cell function or components.
Magnetic stirring of cellular suspensions is generally employed in the art as a preferred method of suspension but usually produces at least some lysis. Furthermore, stirring requires the use of a motor which adds cost and produces heat that may have to be dissipated.
2. Description of the Related Art
U.S. Pat. No. 3,749,369 discloses a magnetic stirring element with a generally ellipsoidal shape. A bar magnet is encapsulated in an inert material. The capsule has a flat base and an upper cavity to hold a measured quantity of an additive. The center of gravity of the element causes it to rotate on its side when subjected to a magnetic field ensuring total dispersion of additive into liquid medium. The device is indicated to be suited for measurement and mixing of components to be blended.
German Patent No. 3,122,018 discloses a device for mixing and stirring of liquid in a hermetically sealed container by the controlled up and down movements of an internal ferromagnetic plate under the action of externally applied magnetic force. The magnetising current is controlled electronically to produce a suitable plate movement pattern for the particular mixture. The device is indicated for use in chemical or medical laboratories where material must be stirred without external contact. The magnetic plate is inserted during initial manufacture of the container. The device is suitable for mixing transfusion blood with ozone in sterile conditions. The blood is mixed carefully with ozone so that no hemolyzing of the blood will occur.
German Patent No. 2,458,904 discloses a magnetic stirring system which comprises a magnetic element inside a container and an externally mounted motor driven magnet. The internal stirrer is a flat plate of rhomboidal shape through which a bar magnet extends perpendicularly to the flat surfaces of the plate. The plate material and the material in which the magnet is encapsulated is non-magnetic and inert to the fluid to be stirred. The system is useful for stirring small quantities of pharmaceuticals, particularly immediately prior to application, little energy is required, friction between the stirrer and the vessel being negligible.
German Patent No. 3,627,132 discloses a magnetic stirring element for miniaturized laboratory apparatus inserted in metal thermoblocks. The element comprises a flat cylindrical core of magnetic material with a high coercive intensity. This is embedded in poly-tetrafluoroethylene such that at least one cavity generates a low effective density and the two diametrally opposed ends have the shape of parallel flats offset from each other by 80-90 degrees. The core comprises preferably a cobalt-samarium alloy. The element effects an adequate turbulence even in parts of slender vessels well above the bottom. The tumbling action is ideal for phase transfer reactions.
U.S. Pat. No. 4,526,046 discloses a fast piston pipette device for microliter and milliliter quantities having a ferromagnetic piece preventing bubble formation as well as washing out dirt and acting as a magnetically-driven stirrer.